1. Field of the Invention
The present invention relates to a charge injection type light emitting device which has a light emitting layer composed of a luminous substance and which can convert field applied energy directly into optical energy when an electric field is applied thereto.
More specifically, compared to conventional incandescent lamps, fluorescent lamps, or light emitting diodes the present invention relates to a charge injection type light emitting device characterized by a large size in area, a high resolution, a small thickness and weight, high-speed operations, and a perfectly solid body and which is used for an electroluminescence (EL) panel that can meet advanced requirements.
2. Related Background Art
An electroluminescence phenomenon of organic material was observed in an anthracene monocrystal by Pope et al. in 1963 (J. Chem. Phys. 38 (1963) 2042), and subsequently in 1965, Helfinch and Schneider successfully observed relatively intense ejection-type electroluminescence (EL) using a solution electrode system having high ejection efficiency (Phys. Rev. Lett. 14 (1965) 229).
Subsequently, as reported in U.S. Pat. Nos. 3,172,862, 3,173,050, 3,710,167, J. Chem. Phys. 44 (1966) 2902, J. Chem. Phys. 50 (1969) 14364, J. Chem. Phys. 58 (1973) 1542, or J. Chem. Phys. Lett. 36 (1975) 345, etc. research was conducted to form organic luminous substance using a conjugate organic host material and a conjugate organic activator having a condensed benzene ring. Cited examples of organic host substances include naphthalene, anthracene, phenanthrene, tetracene, pyrene, benzopyrene, chrysene, pycene, carbazole, fluorene, biphenyl, taphenyl, triphenylene oxide, dihalobiphenyl, trans-stilpene, and 1,4-diphenyl butadiene, and cited activators include anthracene, tetracene, and bentacene.
However, all organic films composed of these organic luminous substances exist as single layers having a thickness exceeding 1 xcexcm, requiring significant electric fields for light emission. Thus, thin film elements formed by a vacuum deposition process were studied (for example, Thin Solid Films 94 (1982) 171, Polymer 24 (1983) 748, Jpn. J. Appl. Phys. 25 (1986) L773).
However, the formation of thin films was effective in reducing drive voltage but failed to provide high-luminance devices that could be appropriately put to practical use.
However, in recent years, Tang et al. (Appl. Phys. Lett. 51 (1987) 913 or U.S. Pat. No. 4,356,429) invented an EL device in which two very thin layers (a charge transporting layer and a light emitting layer) are stacked together between a positive electrode and a negative electrode by vacuum deposition, thereby achieving high luminance at a low drive voltage.
Such stacked organic EL devices have subsequently been actively studied, as described in, for example, Japanese Patent Application Laid-Open No. 59-194393, U.S. Pat. No. 4,539,507, Japanese Patent Application Laid-Open No. 59-194393, U.S. Pat. No. 4,720,432, Japanese Patent Application Laid-Open No. 63-264692, Appl. Phys. Lett. 55 (1989) 1467, and Japanese Patent Application Laid-Open No. 3-163188.
Furthermore, Jpn. J. Appl. Phys. 27 (1988) L269, L713 has reported on an EL device of a three layer structure in which a carrier transporting function and a light emitting function are separated from each other. It also suggests that this device serves to relax constraints on carrier transportation performance in connection with selection of a pigment for the light emitting layer, which determines an emission color, thereby enabling much more free selection. It further suggests that this device serves to improve light emission by effectively containing holes and electrons (or excitons) in a central light emission layer.
Although a vacuum deposition process has commonly been used to produce stacked organic EL devices, it has been reported that considerably bright devices are also obtained by a casting process (for example, Collection 1006 of Manuscripts for the Lecture of the Fiftieth Applied Physics Society (1989) and Collection 1041 of Manuscripts for the Lecture of the Fifty-first Applied Physics Society (1990)).
Furthermore, it has been reported that a substantially high emission efficiency is achieved by a mixed single-layer type EL device formed by a dip coating process using a solution comprised of a mixture of polyvinyl carbazole as a compound having hole transporting capability, an oxadiazole derivative as a compound having electron transporting capability, and cumarin 6 as a luminous element (for example, Collection 1086 of Manuscripts for the Lecture of the Thirty-eighth Applied Physics Related Alliance (1991)).
As described above, organic EL devices have recently been markedly improved, and the possibility of their use for various applications has been suggested.
However, this device has not been studied long, and materials and device forming processes have not been sufficiently studied yet. At present, there still remain unsolved durability problems such as changes over time in characteristics during longtime use and degradation caused by atmospheric gas containing oxygen or humidity. For example, the following problems must be solved:
Organic light emitting devices are of a charge injection type, and their characteristics depend strongly on the amount of carriers injected from both electrodes. Desirably, carriers are always regularly injected from the electrodes (positive and negative) in spite of longtime use.
However, in actuality, partly because of the incomplete electrical matching between organic layers, those of the current carriers flowing through the device which do not particularly contribute to light emission flow from the light emitting layer into the positive electrode during longtime use. Consequently, degradation occurs in the interface between a barrier layer, which contacts with the light emitting layer from the positive electrode side, and the light emitting layer, and also occurs inside the barrier layer, thus reducing the light emission efficiency of the device during longtime use.
The present invention is provided to solve these problems, and it is an object thereof to provide a charge injection type light emitting device that is very durable, i.e., a charge injection type light emitting device that has a long luminance half-value period.
According to an aspect of the present invention, there is provided a charge injection type light emitting device comprising a positive electrode, a negative electrode and an organic film sandwiched between the positive and negative electrodes and comprised of an organic compound, the organic film containing light emitting material that emits light by itself, wherein the energy difference between a vacuum level and the lower end of a conduction band of the light emitting material (Ec, to be referred to as a conduction band level of the light emitting material hereinafter) and the energy difference between a vacuum level and the lower end of a conduction band of a barrier material (Hc, to be referred to as a conduction band level of the barrier material hereinafter) different from the light emitting material meet the condition of Ecxe2x88x92Hc greater than 0.5 eV.
The conduction band levels Ec and Hc may meet the condition Ecxe2x88x92Hcxe2x89xa70.6 eV.
The light emitting material may contain a hydrocarbon compound having a condensed ring (hereinafter referred to as a xe2x80x9ccondensed hydrocarbon compoundxe2x80x9d).
The barrier material may be arranged as a hole transporting layer comprised of an organic compound having hole transporting capability.